Use of BN as a reactant in combustion synthesis of TiN-TiB2 composites under nitrogen pressure

An experimental study on direct formation of TiN-TiB2 composites by self-propagating high-temperature synthesis (SHS) was conducted using compacted samples of titanium (Ti) and boron nitride (BN) powders ignited in gaseous nitrogen. Effects of sample green density, nitrogen pressure, starting stoichiometry of reactant compacts, and diluent content on the degree of conversion and combustion characteristics were investigated. With the addition of TiN as a diluent in the sample, TiN-TiB2 composites with the TiB2 content ranging from 12.5 to 33.3 mol.% were produced at relatively low pressures of nitrogen. Experimental results showed that the TiN-33.3 mol.% TiB2 composite was directly synthesized from SHS reaction of the 1.5Ti + BN sample in argon. Moreover, combustion of a 60% TMD sample made up of 2Ti + BN achieved complete nitridation in nitrogen of 1.48 MPa and yielded a composite of TiN-25 mol.% TiB2. However, due to the melting of reactant compacts in combustion the degree of nitridation was relatively low for the undiluted samples of 3Ti + BN and 4Ti + BN, implying that the addition of diluent TiN was required in order to achieve the full conversion. For the reactant compacts of 60% TMD, the TiN-diluted samples of 2.5Ti + BN + 0.5TiN and 3Ti + BN + TiN reached complete nitridation at 1.14 MPa of nitrogen and produced TiN-TiB2 composites with the TiB2 concentrations of 16.7 and 12.5 mol.%, respectively. For the end products of incomplete conversion, XRD analysis identified not only desired phases of TiN and TiB2, but intermediate compounds of Ti2N and TiB